The literature supports the idea that the plasma membrane of the red blood cell is responsible for the rate-limiting step in oxygen uptake by the cell. We propose to measure the oxygen permeability of the RBC membrane to determine if it limits the rate of oxygen uptake. Red cell ghosts will be loaded with glucose oxidase, glucose, and catalase. This coupled enzyme system will consume O2 in the ghost and maintain low oxygen tension in the cell, thus allowing a rapid and sustained uptake of oxygen that can be accurately measured. The idea that the passage of oxygen through the RBC membrane is rate-limiting for red cell oxygen uptake rests largely on the fact that red cells in the rapid reaction apparatus take up oxygen much more slowly than is calculated for red cells with no membrane resistance to oxygen diffusion. The flow-type rapid reaction apparatus depends on turbulent flow for mixing the reactants. Since suspensions of RBCs are non-Newtonian, whether or not their flow is turbulent in the range of Reynold's Numbers used is open to question. We propose to study the flow behavior of dilute red cell suspensions by means of the laser anemometer to determine the conditions under which there is actually turbulent flow in the rapid reaction apparatus.